Saturn's Moon Titan a World of Rivers and Lakes

By Ker Than |
January 3, 2007 01:01pm ET

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A false-color image of Titan's surface snapped by NASA's Cassini spacecraft on July 22, 2006. The lakes appear darker than the surrounding terrain because of the unique way they scatter radar, which is similar to how water lakes on Earth do it.

Credit: NASA/JPL/USGS

Images shot
last summer by NASA's Cassini
spacecraft provide the strongest evidence yet that Titan, a saturnian moon and one of the most
Earth-like celestial bodies in the solar
system, is dotted with a multitude of liquid lakes.

"At
the time we first announced it, we were like, 'Well, we think these are
probably lakes,' but that was about our level of confidence," said study
team member Ellen Stofan of University College London and Caltech. "I
would say at this point, we've analyzed the data to the extent that we feel
very confident that they are liquid-filled lakes."

Instead of
water, however, the Titan
lakes [image]
are likely filled with methane, and possibly even ethane, organic compounds that
are gases on Earth but liquid on the frigid
surface of Titan.

"It's
going to behave like water," Stofan said about liquid methane. "It's
transparent just the way water is. So if you were standing by the shoreline,
you would be able to see down to whatever pebbles or gunk that was on the
bottom."

Titan is
the only moon in the solar system to have a dense atmosphere with thin layers
of methane and nitrogen clouds--a setup similar to that of early
Earth. Atmospheric methane is destroyed by sunlight over time and must
constantly be renewed. Scientists thus speculated that lakes or even oceans of
methane might exist on, or just beneath, the moon's icy surface and that
evaporation from these liquid bodies was replenishing the atmosphere. The first
confirmation of this thinking came last July
when Cassini's radar spotted more than 75 large, dark patches around the
surface of the moon's northern pole.

"The
lakes are basically black in the [radar] data, which is how a liquid would
behave," Stofan said. Radar data alone wasn't enough, however. A very
smooth deposit of fine soil would also appear black on radar, Stofan explained.

The clincher
that the patches were liquid lakes came from looking at the surrounding
terrain. Some of the patches appeared to be fed by sinuous channels, or "rivers,"
some more than 62 miles (100 kilometers) long. Others appeared to be contained
within rimmed circular depressions, similar to crater lakes or volcanic
calderas on Earth.

"The
morphological evidence points completely away from it being a smooth deposit of
soil or sediment. It's just not consistent," Stofan said. "Combining these
two sets of data, it led us to feel very confident about the interpretation
that they're actually liquid."

In a study
published in the Jan. 4 issue of the journal Nature, the researchers also
suggest that the rivers and lakes are being filled by rainfall from methane
clouds or by seepage from beneath the moon's surface.

"You
can think of all the exotic words you want to call it--a 'methanofer' because
it's not an aquifer. It's a subsurface methane table and not a water
table," Stofan told SPACE.com.

The
researchers predict that as the seasons progress, lakes in the winter
hemisphere should expand while those in the summer hemisphere should shrink or
dry up entirely.

Cassini is
slated to perform 22 more Titan flybys, the next of which is scheduled for
later this month.